The effect of the annealing atmosphere, the annealing temperature etc. on the hydrogen
behavior in several Al-4% Mg alloys during heat treatment, was investigated. The results have shown
that the hydrogen content in the as-cast slab is uniform, while the hydrogen content in the slab tends to
be higher near the surface than in the interior after annealing. Such a tendency becomes more marked
when annealing time is prolonged and Si and Fe content is lowered. The condensation of hydrogen
near the surface can be seen only when it is annealed in a wet atmosphere. When annealed in a dry
atmosphere, the hydrogen content near the surface becomes lower than in the center of the specimen.
The hydrogen in Al-Mg alloys tends to be released to outside intrinsically at temperatures around
400°C and above. It is revealed that oxide film formed on the surface prevents the hydrogen from
being released to outside.